This invention relates to a process for increasing the selectivity of the production of isobutylene in an admixture of C 4. olefins.

The present invention provides a process for producing an isoolefin with high selectivity, comprising: passing a feed comprising an aliphatic hydrocarbon containing from 5 to 20 carbon atoms in the vapor phase at a temperature up to 621°C (1150°F) over a first catalyst composition comprising ZSM-5, ZSM-12 or zeolite Y to produce a first composition comprising at least one normal-olefin of at least 4 carbon atoms in a first product stream, and contacting the normal-olefin with a second catalyst composition comprising a zeolite selected from the group consisting of ZSM-22, ZSM-23, ZSM-34, ZSM-35 and ZSM-48 under a second set of conditions which include a temperature within the range of from 371°C (700 ^β F) to 621 ^β C (1150°F). The four C. mono-olefins, 1-butene, cis-2-butene, trans-2-butene and 2-methylpropene are collectively called butylenes. The term isobutylene is by established usage interchangeable with the nomenclature 2-methylpropene, while the other three isomers are n-butenes. Often they are treated collectively because the four mono-olefins are obtained as mixtures from natural gas and from petroleum refinery processes.

Isobutylene is a desirable reactant for the production of alkylate, an oligomer of petroleum refinery C_-C. off gases, which includes high octane gasoline components, and for the production of

methyl-t-butyl ether when isobutylene is reacted with methanol. In a similar manner, isoamylene can be converted to t-amyl ether. A conventional process for the separation of isobutylene from the other three components involves sulfuric acid extraction or selective adsorption, as the isomers cannot be separated by simple extraction. Acid extraction is cumbersome and includes as an undesirable aspect the oligomerization of the components themselves. Figure 1 is a graph of the plot of the selectivity of the production to iso-olefin vs. conversion of n-butenes. The drawing illustrates the effect of the catalyst on iso-butene selectivity, of 1-butene conversion at 450'C and one atmosphere. The process of the invention comprises catalytic production of a C or a C.+ olefin mixture, including 1-butene, cis-2-butene, trans-2-butene and 2-methylpropene, in the gaseous phase, and contact of that mixture with a catalyst which will convert at least one of the members selected from the group consisting of 1-butene, cis-2-butene, and trans-2-butene to isobutylene and/or isoamylene product, essentially free of oligomers of any of the C. monoolefins under isomerization conditions. Catalytic production of olefin(s) , in accordance with the invention, can yield C . olefin(s) or C.+ olefin(s) , for example, C_ olefin(s) such as amylene in the vapor phase. Catalytic production of the C. olefin mixture, including 1-butene, cis-2-butene, trans-2-butene and 2-methylpropene, is undertaken in the gaseous phase. The reactant for the catalytic production of the C olefin mixture, comprises an aliphatic feed which contains aliphatics having five to thirty carbon atoms-. The aliphatic hydrocarbons can be

acyclic, straight or branched chain, or cyclic, either of which can be saturated or unsaturated, and include alkanes, alkenes, cycloalkanes, cycloalkenes; furthermore, the cycloalkanes and the cycloalkenes may be substituted or unsubstituted by alkyls or alkenyl groups. The aliphatic source may be a Udex raffinate, virgin distillate boiling below 343 ^β C (650 ^β F) , light distillate and/or a naphtha. Typical naphtha feedstock materials for selective cracking are produced in petroleum refineries by distillation of crude oil. Typical straight run naphtha fresh feedstock usually contains 20 to 50 wt % C_-C ₁₂ normal and branched alkanes, 20 to 50% C7+ cycloaliphatic (i.e. naphthene) hydrocarbons, and 1 to 40% (preferably less than 20%) aromatics The C ₇ -C _1? hydrocarbons have a normal boiling range of 65° to 175°C. The process can utilize various feedstocks such as cracked FCC naphtha, hydrocracked naphtha, coker naphtha, visbreaker naphtha and reformer extraction (Udex) raffinate, including mixtures thereof. The catalyst, for the catalytic production of the C. and C.+ olefin(s) , comprises a medium and/or large pore size (5+A) zeolite, supported on a matrix or unsupported. The medium pore size zeolites are shape selective, having a silica-to-alumina ratio of at least 12, a constraint index of 1 to 12 and acid cracking activity (alpha value) of 1 to 15 based on total catalyst weight. When Alpha Value is examined, it is noted that the Alpha Value is an approximate indication of the catalytic cracking activity of the catalyst compared to a standard catalyst and it gives the relative rate constant (rate of normal hexane conversion per volume of catalyst per unit time) . The activity of the standard catalyst, a high activity

silica-alu ina cracking catalyst with a Rate Constant = 0.016 sec ^" is taken as an Alpha of 1. The Alpha Test is described in U.S. Patent 3,354,078; in the Journal of Catalysis- Vol. 4, p. 527 (1965); Vol. 6, p. 278 (1966) ; and Vol. 61, p. 395 (1980) . The experimental conditions of the test used herein include a constant temperature of 538°C and a variable flow rate as described in detail in the Journal of Catalysis. Vol. 61, p. 395.

Representative of the medium pore shape selective zeolites are ZSM-5, ZSM-11, ZSM-12, ZSM-48, MCM-22 and mixtures thereof with similarly structured catalytic materials. Preferably, the zeolite used to produce the olefin(s) is ZSM-5 or ZSM-12. The zeolite ZSM-5 is described in U.S. Patent No. 3,702,886; and the zeolite ZSM-12 is more particularly described in U.S. Patent

No. 3,832,449. The cracking of naphtha in the presence of ZSM-5 and ZSM-12 is the subject of allowed U.S. Patent Application Serial Number 442,806, filed November 29, 1989, now U.S. Patent No. 4,969,987. The catalyst may be in the form of a powder, spheres, beads or extrudates. Supports for the medium pore size zeolites are described below.

The medium pore size zeolite may be used in conjunction with or in admixture with larger pore size zeolites, with pore sizes of at least 7A. Such larger pore size zeolites include zeolites X and Y, dealu - inated Y, ultrastable Y, zeolite beta and zeolite L.

Catalytic production of the C. olefin(s) is undertaken by passing the aliphatic feed, preferably containing high concentrations of naphthenes, over the catalyst in the vapor phase. Catalyst contact with the feed can be undertaken in a fixed bed, moving bed or fluidized bed. The physical conditions of the vapor

phase catalysis for aliphatic(s) conversion to olefin(s) includes a temperature within the range of from 454°C (850°F) to 621°C (1150°F), preferably from 538°C (1000 ^β F) to 593°C (1100°F). The WHSV is from 0.5 to 20, preferably from 2 to 10. The catalyst contact time can range from 0.5 to 10 seconds, preferably from 1 to 5 seconds. The operating pressure is 0-150 psig, preferably 10-50 psig.

The exact distribution and yield of C.s will depend on the operating severity. The C. fraction may be separated from C and C hydrocarbons, which may also be produced, by conventional distillation. However this separation is not essential and is not necessarily preferred.

The catalytically produced C. olefin mixture is contacted with ZSM-23, or zeolites with similar structure, ZSM-22, ZSM-34, ZSM-35 and ZSM-48, under isomerization conditions, to increase the isobutylene content of the composition, and to decrease the content of the C.s other than isobutylene, while maintaining the total C. isomers substantially constant, substantially without oligomerization thereof. Accordingly, the product of the process of the invention is substantially free of oligomerization products of any one of the C. mono-olefins. The catalytic conversion of the C. mono-olefin mixture is undertaken in the vapor phase.

ZSM-22, is more particularly described in U.S. Patent No. 4,556,477; ZSM-23 in U.S. Patent No. 4,076,842; ZSM-34 in U.S. Patent No. 4,086,186; ZSM-35 in U.S. Patent No. 4,016,245; and ZSM-48 in U.S. Patent No. 4,375,573.

As indicated above, the zeolites of the process may be unsupported or supported on a matrix or may be

in the form of a powder, spheres, beads or extrudates. Supports for the zeolites or matrix components include the following:

₃ matrix component particle density (qm/cm ) alumina 3.9 - 4.0 silica 2.2 - 2.6 magnesia 3.6 beryllia 3.0 barium oxide 5.7 zirconia 5.6 - 5.9 titania 4.3 - 4.9

Combinations of two or more of these and/or other suitable porous matrix components, e.g., silica- alumina, silica-magnesia, silica-thoria, silica-alumina-zirconia, etc. , can be employed for a still wider spectrum of density values from which one may select a specific predetermined value as desired. Isomerization of the olefin mixture can be undertaken at a temperature within the range of from

371"C (700 ^β F) to 621 ^β C (1150 ^β F) , preferably from 399 ^β C (750 ^β F) to 566°C (1050°F) , more preferably 371*C to 510 ^β C (700° to 950 ^β F) and most preferably 371 ^β C to 482 ^β C (700° to 900°F). The WHSV is from 5 to 200, preferably 15 to 50. The catalyst contact time can range from 0.01 to 10 seconds, preferably from 0.03 to 5 seconds. The operating pressure is 0-150 psig, preferably 10-50 psig. The cracking catalyst and the ZSM-23 containing catalyst composition can be in admixture.

The process of the invention may be undertaken in a fixed bed, moving bed or fluidized bed. Preferably, C. production and isomerization thereof is undertaken

under fluidized bed conditions, under operating conditions described above. In a preferred embodiment , the two catalyst components are mixed. Alternatively, the process may be undertaken in a fixed bed system. Thus, the catalyst beds including the cracking catalyst component and the ZSM-23 may be in different units or alternatively in sequential beds in a cascade operation. If a fixed bed operation is employed, preferably, it is operated as a cascade operation in which the paraffin feed is converted to C. with the cracking catalyst component and then with the isomerization catalyst comprising the ZSM-23.

Although various amounts of the two sets of catalysts can be used, the isomerization catalyst inventory is preferably less than 5% of the total catalyst inventory; it is preferred to operate at an isomerization catalyst make up rate of greater than zero (0) and less than 0.3 weight percent of the total catalyst inventory per day.

The selective cracking conditions include total pressure up to 500 kPa and reaction temperature of 454°C (850°F) to 621°C (1150°F), preferably at pressure less than 175 kPa. Cracking reaction severity can be maintained by employing a weight hourly space velocity of 1 to 20 (WHSV based on active catalyst solids) ; and contact time less than 10 seconds, usually 1-2 seconds. The conversion of n-butene to iso-butene over ZSM-23 at atmospheric pressure, high WHSV, and 538°C (1000°F) occurs with no significant oligomerization to heavier molecules. The ZSM-23 isomerization of n-butene(s) is favored by low reactant partial pressure and high operating temperature in a cracker process. In such an embodiment, preferably the ZSM-23 containing catalyst is added to the cracker in

short time intervals or continuously. The ZSM-23 catalyst can be added to the cracker unit at any location in the riser, transfer line, or reactor cyclones.

EXAMPLES

In Table 1, the results of passing 1-butene (152 Torr); over HZSM-23 (alpha=19) (.06013 g m s) under the conditions set forth are set forth.

TABLE I

Press (Psig) 8 Temp (°C) 501 Flow (CC/Min) 200 WHSV

WEIGHT PERCENT IN PRODUCT STREAM

CIO 0.238 0.177

C20 0.031 0.022

C2= 0.281 0.199 C30 • 0.014 0.008

C3= 1.351 0.971

I-C40 0.158 0.110

N-C40 0.561 0.461

1-C4= 16.56 17.915 I-C4= 34.474 30.874

TR-2-C4= 26.935 28.726

CIS-2-C4= 18.518 19.904

N-C50 0.000 0.000

3M-1-C4= 0.000 0.000 1-C5= 0.000 0.000

TR-2-C5= 0.119 0.076

CIS-2-C5= 0.044 0.025

TERT-C5= 0.654 0.476

C6= 0.079 0.055 C7+ 0.026 0.000 C1-C5 PARFNS 1.002 0.778

C2= 0.281 0.199

C3= 1.351 0.971

C4= 96.443 97.419 C5= 0.817 0.577

C6= 0.079 0.055

C7+ 0.026 0.000

Conversion of N-C4= 38.031 33.455 I-C4= 34.474 30.874

Selectivity to I-C4= 90.647 92.285

The addition of ZSM-23 catalyst increases the presence of all iso-olefins in the reaction effluent, particularly isoamylene and isobutene.